In many applications it is desirable to capture imagery in one to several limited bands of Thermal Infrared Radiation (TIR) with cameras sensitive to the MidWave Infrared Region (MWIR) generally defined as encompassing wavelengths from 3 to 6 micrometers (μm) and/or the Long Wave Infrared (LWIR) generally defined as encompassing wavelengths from 7 to 14 μm. For example, instead of imaging over the full detector response of an LWIR camera one may wish to use a filter to limit the response to wavelengths between 8 to 9 μm or 10.5 to 11.5 μm, etc. Most commercial camera systems designed to collect multi-spectral imagery (i.e., images at two to several discrete wavelength bands) utilize cryogenically cooled quantum type detectors and, as a consequence, are expensive. Such systems use a cooled infrared focal plane array (IRFPA) and cooled individual optical band pass filters or multiple filters in a cooled filter wheel. Another class of TIR cameras exists that use uncooled detectors, such as microbolometer (or amorphous silicon and other types) cameras, which are also sensitive to the thermal infrared region of the spectrum, typically wavelengths from about 7 μm to 14 μm. These cameras are a low cost alternative to the expensive cooled detectors, and are very popular in industrial, law enforcement, security, military, and research applications. One reason the cost of the microbolometer camera is so low, relatively speaking, is that microbolometer sensors operate with little or no cooling as opposed to the cooled quantum type detectors mentioned previously, which require a good degree of cooling, often to 80K or below.
In many applications it is desirable to capture imagery in one to several limited slices of the radiant energy spectrum, for example 8 to 9 μm only, or 10.5 μm and above only, or 10.5 μm and below only, and the like. Band-pass, long-pass, short-pass, or notch filters are available from commercial sources for selectively slicing up the radiant energy spectrum in that way. Such filters are referred to as “spectrally selective.” However, using those commercial spectral filters with microbolometer cameras poses a challenge since the filters transmit only over a certain portion (e.g., the band-pass) of the spectrum, but reflect and/or emit radiation at all other regions of the spectrum. The relationship between reflection and transmission is described in this situation by Kirchoff's Law, which states that in thermal equilibrium τ+ρ+α=1 and α=ε. The terms in the foregoing equations represent transmission (τ), reflection (ρ), absorption (α), and emissivity (ε), respectively. Typically spectral filters used in the thermal infrared have relatively low absorption coefficients, so the radiation that is not transmitted is primarily reflected. By the same token, in thermal equilibrium, the filter emits as much thermal radiation as it absorbs.
If a band-pass filter is placed in front of a microbolometer camera the image is degraded by parasitic background radiation from both the emission of the filter and the reflection off of the filter. With the filter placed at normal incidence, that is, perpendicular to the optical axis, the reflected radiation is from the warm detector, which in the case of a microbolometer is often significantly higher than room temperature. That effect is not a problem with the more expensive quantum-type cooled IRFPAs, since those detectors are maintained at a cold enough temperature to avoid emission of a significant level of radiation. In such cameras the filter is also cold. Thus neither reflected nor emitted radiation is present in cooled quantum-type FPAs.
The graph FIG. 1, labeled Narcissus effect is an example of the result obtained from placing a band pass filter that passes wavelengths between 8 μm to 9 μm and reflects other wavelengths in front a microbolometer camera. The graph provides a comparison between (a) the calculated “out of band” thermal radiation in the 7 to 8 μm range, and the 9 to 14 μm range from the microbolometer at different temperatures (shown on the X-axis) that is reflected onto itself (Narcissus effect) from the surface of the filter, with (b) radiation reaching the microbolometer from a scene at the designated temperature (Y-axis) using f/2 optics and passing through the 8 to 9 μm wavelength band pass filter. The graph shows that the Narcissus effect due to a microbolometer that is maintained at 30° C. is equivalent in terms of the incident radiation at the FPA to watching a target that is about 1,170K in temperature.
As a consequence, a microbolometer camera with a band pass filter positioned in front of the camera has a relatively large amount of parasitic radiation that is not useful for imaging. The parasitic radiation may be subtracted using an offset correction; however, the process may be only partially effective because of the introduction of radiation that degrades the image quality by limiting the dynamic range of the detector to avoid saturation. Additionally, for systems using multiple filters (such as in a filter wheel), the reflection from different filters can differ both in terms of intensity and spectral content. Therefore, a different offset is needed for each filter.
More specifically, patents representative of the background art, which are herein incorporated by reference in their entirety, include: U.S. Pat. No. 6,023,061 disclosing a camera having a two dimensional uncooled FPA, a compound lens for imaging IR radiation onto the FPA, and a means for adjusting the distance between the lens and the FPA to adjust focus; U.S. Pat. No. 6,853,452 disclosing a remote sensor for use as a handheld, mobile or stand-alone sensor that has first and second optical paths, light collecting optics, and a sample filter assembly positioned in a first optical path for passive, remote sensing of chemicals; and U.S. Pat. No. 6,515,285 disclosing methods and apparatus for compensating a radiation sensor for ambient temperature variations.
In addition, patent application publications representative of the background art, which are herein incorporated by reference in their entirety, include: US 2007/0176104 disclosing a process and system for a medium wave infrared (MWIR) uncooled microbolometer focal plane array (FPA) utilizing standard silicon processing techniques; US 2004/0223069 disclosing a tunable imaging sensor includes a housing with four lenses mounted on a front side; and US 2007/0120058 disclosing an uncooled infrared sensor utilized for fire fighting, surveillance of a border or any desired area, and limb sounding.
Accordingly, there is a need for a multi-spectral imaging system and apparatus that provides expanded capabilities and additional advantages over conventional systems, such as improving image quality, and reducing and or minimizing parasitic and background radiation.